1. Technical Field
The present disclosure relates in general to well control and intervention systems and methods. More particularly, the present disclosure relates to systems and methods for running casing in wells drilled with dual—and/or multi-gradient mud systems.
2. Background Art
Drilling operations that encompass various methods of drilling a subsea well with two different fluid densities or mud weights (Dual Gradient Drilling Systems) have been publicized. See for example, U.S. Pat. Nos. 6,536,540; 6,843,331, and 6,926,101. Previous industry projects have developed and are developing drilling methodologies to safely employ the technology. Benefits of a dual gradient drilling system include reduction of the hydrostatic pressure in the well annulus above the bottom or at a previous casing point while simultaneously maintaining a higher equivalent hydrostatic pressure at the bottom of the hole. There are also known so-called “multi-gradient” mud systems, in which beads having density less than a heavy mud are added to a portion of the heavy mud present in a marine riser. Such mud systems are known (using incompressible beads), for example, from U.S. Pat. Nos. 6,530,437 and 6,953,097. Finally, there have been disclosed so-called “variable density” mud systems employing compressible beads, such as described in published U.S. Pat. App. Nos. 20070027036; 20090090559; 20090090558; 20090084604; and 20090091053. The methods and systems described in the present disclosure are applicable to all of these different types of mud systems, and are generally referred to herein simply as “dual gradient mud systems.” The patent documents referenced in this paragraph are incorporated herein by reference for their disclosure of multi-gradient and variable gradient mud systems.
Although previous research projects have developed equipment and methodologies to drill wells with dual gradient mud systems, systems and methods to run casing in these wells, with the dual gradient mud systems in place, have not been developed previously. Due to very tight margins between the mud weight needed to control pressures in deep subsea wells and to simultaneously not induce sufficient pressure to cause formation breakdown in the annulus above total depth with resulting fluid losses to the formation between a casing shoe and total depth, what remains needed are systems and methods that allow casing to be run with surge reduction equipment thereby avoiding inducing fluid losses that would jeopardize the well and the primary cement job in particular.
Current practice for running casing in wells drilled with dual gradient muds is reflected in U.S. Pat. No. 6,328,107, which discloses a method for controlling the pressure at the base of a gas-lifted riser during casing installation. Prior to casing installation, drilling fluid is displaced from the riser and the riser is filled with seawater. During casing installation, the riser base pressure is monitored, and the height of seawater in the riser is adjusted to compensate for increases in the riser base pressure. The riser base pressure is thereby maintained substantially equal to the seawater pressure at the base of the drilling riser throughout installation of the casing.
It would be advantageous if systems and methods could be developed that safely allow casing to be run into wells (and cemented) with a dual gradient mud system efficiently and with improvements in well control. It would also be advantageous if the systems and methods would allow management of the dual gradient mud system and maintain the benefit of having a dual gradient mud system in the well. The systems and methods of the present disclosure are directed to these needs.